Neuronal morphogenesis is a highly regulated process that ultimately depends on the remodeling of neuronal cytoskeleton in response to extracellular cues. Most previous studies of neuronal cytoskeleton focus on the regulation of actin filaments by extracellular cues. Very few studies have been done to investigate how neuronal microtubules (MTs) are regulated. Regulation of MTs is involved in every step of brain development, such as asymmetrical cell division of neural precursor cells, neuronal migration, axon growth and guidance, dendrite development, and synaptogenesis. Mutation of many MT regulating proteins during development is often associated with mental illnesses, underscoring the importance of MT regulation in normal brain development. Thus, our long-term goal is to understand how extracellular signals governing neuronal morphogenesis are transduced into MT reorganization in neurons that is necessary for proper axonal and dendritic development. Our preliminary study has revealed a novel mechanism by which MT plus end tracking proteins, CLASPs, regulate mammalian axonal and dendritic growth. We found that, unlike other +TIPs that only track MT plus ends, CLASPs display dual bindings to either the plus ends or along the sides of MTs (or MT lattices) in neurons. Functionally, we show that this unique dual MT binding behavior of CLASPs allows them to differentially regulate MT organization and axon growth in different neurons. In regenerating sensory neurons, CLASP mainly bind to MT plus ends and function to support fast axon growth. In contrast, CLASP in developing cortical neurons show increased binding along the side of MTs and act to restrict axon growth. In addition to restricting axon growth, we also provide evidence that CLASPs function to support the development of cortical neuron dendrites. Interestingly, axon guidance cue Slit also functions to repel axons and in the meantime promote dendritic growth. Because CLASP has been placed downstream of Slit to mediate axon repulsion in Drosophila, we hypothesize that CLASPs, with their unique dual MT binding property, may be converging targets of Slit-Robo signaling to regulate mammalian axonal and dendritic development. Thus, the overall goal of this study is to elucidate the role of CLASP in regulation of neuronal morphogenesis in response to extracellular cues during cortical development. To test this hypothesis, we will 1) elucidate the molecular mechanism by which CLASPs regulate MTs to control axon growth and dendritic development, 2) determine the roles of CLASPs in Slitmediated axon repulsion and dendritic growth using in vitro axon guidance assay and cell cultures, and 3) determine the in vivo roles of CLASPs in cortical neuron axon growth/guidance, and dendritic development during cortical development using in utero electroporation. This study will reveal novel molecular mechanisms by which extracellular cues regulate MTs to control neuronal morphogenesis. PUBLIC HEALTH RELEVANCE: Neuronal morphogenesis, including axon growth, guidance, and dendrite growth are key molecular events underlying the formation of the neural circuit during development, or neural repair after injuries. Mistakes in these processes during development are believed to cause many neurodevelopmental disorders. Therefore, our proposed study will not only help us understand how neural circuits form during development, but also provide valuable information of how to promote neural regeneration following injuries.